While having applicability in other reinforced concrete structures, the present invention is primarily concerned with the construction of reinforcing cages for concrete pipe. Such pipe typically may be of a diameter of anywhere between 1 and 16 feet or more and is conventionally constructed in sections of axial lengths normally between 4 and 16 feet. The most frequently encountered pipe is constructed with a bell mouth at one end dimensioned to internally receive the end of an adjacent pipe section so that sealed joints between adjacent pipe sections can be achieved.
Because concrete is relatively weak in tension, it is standard practice to reinforce such pipe with a grid-like wire reinforcing cage which is embedded in the concrete during the molding or formation of the pipe. The cage consists in general of a grid-like wire mesh or lattice formed into an overall shape corresponding generally to that of the pipe section and dimensioned so that the cage is completely embedded within the concrete in the completed pipe. Many examples of such cages are found in the prior art.
Because an assembled reinforcing cage is awkward to transport in any substantial numbers, such reinforcing cages are almost invariably constructed at the pipe-making plant. Nearly all such cages are constructed by one of two methods. In the most frequently employed method, a wire mesh fabric, purchased in rolls or flat sheets, is rolled or formed into the desired tubular shape and welded along an axially extending seam line into the desired tubular configuration. This method presents certain problems in cage sections which must be formed with a bell mouth or spigot because the bell mouth or spigot portion of the cage must be of a different diameter than the main body portion.
In another frequently employed method, axially extending wire rods are laid upon a form and a circumferentially extending wire is helically wrapped around the axial wires and welded at each intersection of the axial and helical wires.
While a reinforcing cage of cylindrical configuration is normally employed on relatively small diameter pipe, the stresses and economics encountered in larger diameter pipe, particularly with relatively thick walled types, are such that cages of more refined and complex construction are required. When a pipe of circular transverse cross-section is laid in place upon its side, the loads applied to the pipe are such as to cause the pipe to tend to assume a flattened elliptical transverse cross-sectional shape. The stresses present are substantially greater at the top and bottom portions of the pipe, while the stresses present in the opposite side portions, at the 3:00 o'clock and 9:00 o'clock positions are substantially less. Further, the concrete is in compression at the outer surface of the pipe at the 12:00 and 6:00 o'clock positions and in tension at the inner surface at these two positions, while the reverse is true at the 3:00 o'clock and 9:00 o'clock positions. To compensate for this latter effect, the transverse cross-sectional configuration of the cage is frequently formed in an elliptical cross-section which is oriented within the pipe so that the cage is closer to those surfaces of the pipe where tension stresses are present. The variation in magnitude of stresses between the top and side portions of the pipe is frequently counteracted by adding additional reinforcement to the cage in those regions where the larger stresses are encountered.
Where thick walled pipe is employed, two or more concentric cages may be employed, the inner and outer cages frequently being coupled to each other by radially extending tie rods.
From the foregoing, it is believed apparent that construction of the cages of the more complex nature can require tooling, techniques and design talent which are not always readily available in a plant whose primary function is the production of concrete products. Manufacture of cages at specialized plants where production volume could support specialized tooling and design facilities has not been feasible because of the cost of transporting completed cages. Conventional or prior art cages of the same size cannot be nested or compacted for transportation and thus present an inordinately high bulk to weight ratio for shipping purposes in addition to the possibility of deforming the cage during shipment and handling.
The present invention is especially directed to a cage construction technique by which the construction of complex cage configurations or cages with bell mouth or spigot sections is simplified and which enables a more efficient use of cage material by employing different gauges or patterns of wire at various regions within the cage in accordance with the local stress requirements to be met, and which further enables shipment of prefabricated cage sections in compact self-braced nested bundles.